Electrical power for an integrated circuit (IC) is typically supplied by one or more direct current (battery) power sources, such as a pulse width modulation (PWM)-based, DC-DC converter. This type of converter contains a PWM signal generator that supplies a synchronous PWM signal to a switching circuit driver. The switching circuit drive, in turn, controls the on-time and off-time of electronic power switching devices (such as a pair of FETs connected between a pair of power supply rails). A common node between the two FETs is coupled through an inductor to a load reservoir or output capacitor, with the connection between the inductor and the capacitor serving as an output node from which a desired (regulated) DC output voltage is applied to the load.
FIG. 1 diagrammatically illustrates a conventional soft start circuit that may be used with such a DC-DC converter. As shown therein, an input reference voltage Vref is coupled to a current limiting (at ILim) Gm Amplifier buffer 10, the output of which is coupled to the non-inverting (+) input 21 of an error amplifier 20 and to a soft start capacitor 30, which is referenced to ground. A feedback resistor 40 is coupled between the output 23 and the inverting (−) input 22 of the error amplifier. The output 23 of the error amplifier is coupled to a pulse width modulator (PWM) 50. The output of the PWM 50 is coupled through an inductor 60 to an output node 65, to which an output capacitor 70, referenced to ground, is coupled. The voltage at the output node 65 is fed back through a feedback resistor 80 to the inverting (−) input 22 of error amplifier 20. A switch 90 is coupled in parallel with the soft start capacitor 30 and operates as follows.
When the DC-DC converter is disabled, the switch 90 is closed, so as to short the capacitor 30 to ground. When the converter is enabled, switch 90 is opened, so that the voltage across the soft start capacitor 30 ramps up from its initial voltage of zero (0) volts to the reference voltage Vref, at a constant charging rate of dv/dt, set by the ratio of ILim/Csoft. The voltage across the output capacitor 70 will follow the voltage ramp across the soft start capacitor 30.
In this circuit, if the output capacitor 70 is charged up at the time the converter is enabled (for example, if the converter is disabled and then reenabled before the output capacitor 70 is discharged), the converter will attempt to discharge the voltage across the output capacitor Cout to the soft start voltage of nearly zero volts across the soft start capacitor 30. If the circuit is not designed properly, the resulting current could be excessively large and may damage components. In addition, circuit operation is inefficient, as it discharges the output capacitor before charging it.